His last small-screen appearance was a light-hearted affair - but this time Ben Affleck is tackling far more serious material.

The actor recently traveled to the Congo - where four million people have been killed in the decade-long conflict, according to the International Rescue Committee. And Thursday night, ABC’s Nightline will air a special documentary on his eye-opening journey.

“Congo is in terribly shape,” Affleck (who has traveled to the country three times in the last eight months) says in the segment. “What’s going on here in the this country is a humanitarian disaster.”

Affleck presents the footage as a personal tale, and doesn’t identify himself with any particular relief organization.

“I don’t think people respond particularly well to being hectored, or being made to feel guilty, or being made to feel as though they’re kind of callow or insensitive for the life they’ve been leading up until then,” he told the Associated Press.

All the star power in the world couldn’t keep the Lakers from blowing a 24-point lead in their devastating loss to the Boston Celtics Thursday night at L.A.’s Staples Center.

“Good for Boston,” Will Smith graciously conceded to PEOPLE after the Celtics’ 97-91 win. He was followed to the exits by a grim-faced Jack Nicholson, a dejected Penny Marshall and disconsolate Justin Timberlake.

But the crowd - which included L.A. supporters Denzel Washington, Tobey McGuire, David Beckham, Nicky Hilton, Ryan Seacrest, Randy Jackson, Dustin Hoffman and Fergie - did contain a few Boston fans.

“It’s a good year for comebacks,” he told PEOPLE. “For the Celtics and the New Kids on the Block.”

Michelle Kwan was there to cheer on the Lakers - and to admire the players’ fit physiques. “They’re all good-looking,” said the L.A. native. “They have nice bodies …. And then you see that passion for the game.”

But her beloved Lakers are in a tight spot now, down 3-1 in the NBA championship series. (The Celtics need just one more win to clinch the title.)

“I think we’ll win it all in game six in Boston next week, and then we’ll celebrate the night,” Wahlberg predicted.

But would his brother Mark (who’s been - gasp! - rooting for the Lakers lately) join in the fun? “Right now, I think his blood is turning purple and gold a little but,” Donnie admitted, referring to the Lakers team colors. “We’re gonna have to fix that!”

They’re the hottest new couple in Hollywood, and now Ben Affleck is speaking out about how he hooked up with late-night host Jimmy Kimmel for their parody video response to Matt Damon and Sarah Silverman - as well as the secret bathtub scene that got cut.

Affleck tells EW.com: “I remember seeing Matt’s %26#91;video%26#93; and thinking to myself, ‘Oh, it would be funny to do one with me and Jimmy.’ They called and immediately I was like, ‘Yes, it should be this idea …,’ and they had the same idea more or less already. I have a lot of respect for the group over there - they’re really good comedians - so I just committed to it.”

The actor says the singing was more difficult than wearing a pair of short shorts, but he acknowledges, “You couldn’t really do it half-speed. You had to be painting toes and blow-drying hair. At one point we were going to take a bath together, but the bubbles couldn’t get high enough, so that ended up getting scrapped.”

As threatened, Jimmy Kimmel has blasted back at girlfriend Sarah Silverman in the wake of her recent revelation - in an hysterical music video - that she and PEOPLE’s Sexiest Man Alive Matt Damon are intimate.

Her song: “I’m F—ing Matt Damon.” Kimmel’s song of revenge, unveiled on his ABC show Jimmy Kimmel Live Sunday night immediately after the Oscars: “I’m F—ing Ben Affleck.” (Watch the video here.)

“Matt, Sarah, this is for you,” said Kimmel, noting that Silverman and Damon’s music video has been seen by about 8 million people on ABC.com and YouTube.

And the hilarious new clip - in which the two are seen giving each other pedicures before Affleck tweaks Kimmel’s bare chest - isn’t just a duet: It’s an all-star performance on par with “We Are the World.”

When it comes to backup singers, Kimmel somehow managed to round up a who’s-who of the entertainment industry, including Don Cheadle, Ashlee Simpson, Robin Williams, Cameron Diaz, Huey Lewis, Christina Applegate, Joan Jett, Macy Gray, Benji and Joel Madden, Lance Bass, Josh Groban and Harrison Ford - who blows the new couple a kiss.

Costarring Brad Pitt

Even Brad Pitt makes a cameo, albeit in a non-singing role. He plays a FedEx deliveryman who brings a cake of congratulations to Kimmel and Affleck.

Kimmel and Affleck also stand nose to nose in the video, and all but kiss. As Robin Williams rhapsodizes, “This is not a man crush.”

“The reason I did it like this, I didn’t want my parents finding out from the tabloids,” Kimmel said after showing the video.

Affleck said his wife, Jennifer Garner, didn’t take the news very well. “Thank God my daughter is too young %26#91;to understand%26#93;,” added the actor, referring to 2-year-old Violet.

Retorted a straight-faced Kimmel, “Well, she’s our daughter now.”

Jimmy Kimmel really does love Ben Affleck.

The talk show host - who memorably declared “I’m F—king Ben Affleck” in a music video Sunday night - cannot stop singing the actor’s praises.

“Ben could not have been more sporting,” he told PEOPLE of his video costar. (The song was designed as a retaliation against Kimmel’s girlfriend Sarah Silverman for her clip, “I’m F—king Matt Damon.) “He told us, ‘Listen, if we’re going to do this, we’ve got to do it right. I don’t want you guys to worry about my image.’ ”

There are also non-singing cameos by Brad Pitt and Harrison Ford, who blows the new couple a kiss before driving off in his convertible - revealing a “Honk if you’re f—ing Ben Affleck” bumper sticker.

“That was his real car,” Kimmel says of the legendary Indiana Jones star. “He’s still got that %26#91;sticker%26#93; on there!”

So what did Silverman think of the new song? “I suspected Jimmy was on the down low for a long time now,” she deadpanned to the New York Daily News. “I wish I could say I was surprised by this.”

If she’s plotting another salvo, Kimmel isn’t quaking in his cut-off denim shorts. “I suppose it’s possible that Sarah and Matt Damon are huddled in a cave somewhere planning their revenge,” he said. “But one thing I know is, if they are, they won’t tell me about it!”

• Ben Affleck, hanging with pal Sheryl Crow during a party at New York’s Soho Grand penthouse following a

Cinema Society screening of his film Gone Baby Gone. Guests at

the bash (including Casey Affleck and Patricia Clarkson)

enjoyed a spread of comfort food - meatloaf, mashed potatoes, Brussels sprouts - in a comfy setting with big beds and sofas. And a thin looking Affleck shared a diet secret with PEOPLE earlier in the night. “%26#91;I%26#93; just lay off the bread,” the actor said. Back at the penthouse bash, Clarkson raved about the man of the hour. “Ben is a wonderful guy,” she told us. “He made a great movie.”

• Newly engaged Rose McGowan, indulging a sweet tooth at Beverly Hills bakery Crumbs,

where she picked up cupcakes in flavors including coconut, pi%26#241;a

colada, caramel apple and vanilla. (No sign of her director fiancé Robert Rodriguez, though.)

• Jessica Alba, enjoying a poolside dinner at blue

on blue in Beverly Hills’ Avalon Hotel.

• Wilmer Valderrama, taking in an explosive Kanye West performance from atop a banquette inside a T-Mobile Sidekick LX

party at the Clubhouse at the Griffith Park’s Harding and Wilson Golf Course

in Los Angeles. The rapper offered attendees (including Carmen Electra and Shia LaBeouf) a long performance

dominated by tunes from his latest disc, Graduation (and blinding strobe lights that had audience members joking they’d have seizures). Meanwhile, Electra watched front-and-center, shimmying from inside her banquette. Jeremy Piven alternated his attention between his male friends and a group of women who congregated around him, and LaBeouf enjoyed the company of a few female partygoers, too.

• Ron Howard, playing his favorite role - doting grandpa - during lunch at AMMO in Hollywood with his wife, daughter Bryce Dallas Howard, her husband Seth Gabel (Dirty Sexy Money) and their 8-month-old son.

• By JENNIFER GARCIA, MAUREEN HARRINGTON, SCOTT HUVER, MARISA

LAUDADIO, ALYSSA SHELASKY and JEFFREY SLONIM

Wednesday’s Caught in the Act >