Ok, which characters where they?
Andre 3000 in Be Cool. He was also in a few episodes of The Shield.
Big Daddy Kane, Tone Loc, and Isaac Hayes were all in Posse.
Which characters were they?
McGraw plays Sandra Bullock’s husband. They are the adoptive parents of Michael Oher.
In Mystery Train Joe Strummer plays Johnny aka “Elvis.” He is the “brother-in-law” of Steve Buscemi’s character. They appear in the segment “Lost in Space.”
I got a good one: in 1994, when Mark Wahlberg was still just “Marky Mark.” His first feature film role was in Renaissance Man, as one of the less-than-brilliant army recruits Danny Devito is teaching. There’s a Hamlet-themed hip-hop/rap routine in the movie, but Wahlberg’s rhythmically-challenged character just drums a desk (poorly) in the background.
If you watch the credits, you’ll see that Wahlberg actually wrote the Hamlet Rap. He’s also featured on three songs on the soundtrack, but he doesn’t perform in the movie.
How about Clint Eastwood? He’s a singer; just listen to him sing in Paint Your Wagon!
It was also one of Courtney Love’s first film roles. Especially creepy given what happened to Cobain 8 years later.
Chris Isaak was in a blink-and-you-miss-it role as a SWAT team leader in “Silence of the Lambs.” He has some lines but his character isn’t even named.
I always wondered how many takes it took him to sing “you got the touch” so poorly in Boogie Nights knowing that he knows how to carry a tune.
Dave Matthews as Otis in Beacuse of Winn-Dixie. His character did sing some.
Agreed. I thought he was understatedly excellent in Something the Lord Made, about a little-known but important mid-20th-century medical researcher.
If I recall correctly, didn’t Tom Petty make an appearance in The Postman which had nothing to do with him as a musician?
A few more I thought of:
Down By Law, Short Cuts, The Fisher King, The Imaginarium of Dr. Parnassus.
I know he’s been in more stuff that I can’t think of.
Actually they do make reference to the fact that he “Used to be someone famous” although his character is not listed in IMDB as Tom petty, just as “Mayor of Bridge City”
Thank you for the name of that excellent film. I saw the first 3/4 on motel cable TV, never saw the title, but what an outstanding movie with an exceptional cast, including Mos Def–have no idea who he is, had no idea who he was except that wonderful young, black actor from that Alan Rickman movie. He needs to quit rappin’ and pound out a serious film career. 
Ludicris, too. Outstanding in one of those SVU episodes.
Nitpick: Mark Wahlberg was a rapper, not a singer. 
Mos Def/Yasiin Bey also had a recurring role in Season 6 of Dexter.
I don’t get what the OP means by “no big deal was made”, but if he is looking for singers in supporting roles where they don’t sing then Cry Baby is notable for being a musical in which Iggy Pop appears but IIRC does not sing at all.
It was obvious stunt casting, but in The Adventures of Ford Fairlane, Wayne Newton didn’t sing.
Neither did Robert Goulet in Naked Gun 2½: The Smell of Fear. (They couldn’t get Wayne Newton.)
And don’t forget that Meat Loaf showed up for dinner in The Rocky Horror Picture Show. (He also sang. Hot patootie, bless my soul!)
Also in Fight Club was a young, pre-Oscar Jared Leto (although the first album from 30 Seconds To Mars was released 3 years later). Brad Pitt looks right at Leto when he says the line: “We’ve all been raised on television to believe that one day we’d all be millionaires, and movie gods, and rock stars.”
If cameos count, Joe Strummer, Mick Jones, and Paul Simonon all appeared as “Street Scum” in The King of Comedy (along with Ellen Foley, who sang with Meat Loaf on Paradise By The Dashboard Light).
Nitpick accepted, but I still feel like a rapper would be able to croak out something better than this, which if I recall he did fine in the another movie where he played a heavy metal singer.