Between the substance abuse, the sex tape, and the stint on “Celebrity Rehab,’’ it’s been a tough 10 years for Tom Sizemore. But the actor known for his roles in “Saving Private Ryan’’ and “Black Hawk Down’’ says he’s doing better by the day. Nearly three years sober, Sizemore is in town for a Saturday screening of “Slumber Party Slaughter’’ at the Boston International Film Festival. “I’ve regained my confidence as an actor,’’ says Sizemore, who plays an actor with a substance abuse problem. (How’s that for cinéma vérité?) “The character’s not exactly sympathetic, but you can see he’s a good guy.’’ The film was written and directed by Rebekah Chaney, the grand-niece of Lon Chaney, who told us she wrote the part for Sizemore while watching him struggle with his demons on “Celebrity Rehab.’’ “It broke my heart,’’ she said. “Mr. Sizemore has a sensitivity you don’t often find. We’ve made something very cool here, a cult classic, we hope.’’Christopher Muther of the Globe staff contributed. Read the Names blog at www.boston.com/namesblog. Names can be reached at email@example.com or at 617-929-8253.