1989 marked a career turning point for the young actor as Sizemore appeared in an ABC TV-movie, small roles in four features, including playing a glib con who befriends Sylvester Stallone in the slammer in "Lock Up" and an angry wheelchair-bound vet in Oliver Stone's "Born on the Fourth of July", and memorably as the love interest of Colleen McMurphy (Dana Delany) in a story arc on the Vietnam drama "China Beach" (ABC).
Sizemore brings a spontaneity and humor to his often intense, fast-talking characters (usually cops or cons on the fringe), that raises them above the cliches they could have been. He stood out in several of his five feature credits in 1993. As the wisecracking cop partner of Chris Penn, the actor held his own against some accomplished scene-stealers in "True Romance". He shone in "Hearts and Souls" in a more substantial supporting role as the amiable hood/guardian angel of Robert Downey Jr. Sizemore also registered as a stolid Bat Masterson allied with Kevin Costner's "Wyatt Earp" and expanded his rep playing an obsessive detective on the trail of a murderous couple, reteaming with director Oliver Stone in "Natural Born Killers" (both 1994). He remained busy in 1995 as "Devil in a Blue Dress" found him playing an enigmatic "fixer" who draws P.I. Easy Rawlings (Denzel Washington) into a dangerous case. "Strange Days" cast the actor as a sleazy detective while "Heat" had him as one of Robert De Niro's partners-in-crime.
Sizemore has publicly credited De Niro for helping him overcome a drug habit. Following the completion of "Heat", De Niro staged an intervention and Sizemore entered a rehab clinic. Now drug-free, he managed to land the lead as a slightly cynical cop investigating a mysterious murder at a Chicago museum in "The Relic" (1997). After packing on the pounds, Sizemore offered a chilling portrayal of reputed mobster John Gotti in the 1998 NBC miniseries "Witness to the Mob", executive produced by De Niro. Later that year, he had his biggest breakthrough to date as Sergeant Hovarth, the loyal right-hand man of Tom Hanks' Captain John Miller in Steven Spielberg's startlingly popular and roundly acclaimed World War II epic "Saving Private Ryan". The actor received exceptional notice for his multifaceted performance in this brutally violent yet spellbinding account of the rescue of the titular missing soldier from behind enemy lines. An actor personally and professionally acquainted with Robert De Niro, and one who has worked well with such big time directors as Spielberg, Oliver Stone, Kathryn Bigelow and Michael Mann, Sizemore finally joined up with the legendary Martin Scorsese, with a scene-stealing performance as a wild ambulance driver in "Bringing Out the Dead" (1999).
He received additional acclaim and notice for his gripping turn in the HBO original drama "Witness Protection" as a gangster turned government witness and target. Keeping a steadily high profile, Sizemore was next featured with the lead supporting role as a fight promoter in Ron Shelton's "Play It to the Bone" (1999), a boxing film pitting two fighter friends (Woody Harrelson and Antonio Banderas) against each other in the ring. The following year the busy actor co-starred with Val Kilmer in the sci-fi thriller "Red Planet" (2000) and was also in the ensemble of "The Match", a Scotland-set soccer drama. In 2001, he appeared in two high-grossing war films "Pearl Harbor" and "Black Hawk Down." He landed a television drama series leading role as a LAPD captain in the CBS show "Robbery Homicide Division" in 2002 and then co-starred with Morgan Freeman in the supernatural thriller feature "Dreamcatcher" (2003), playing a refreshingly heroic and untroubled role.
However, Sizemore's personal life soon appeared to resemble that of some of his more troubled characters: the actor, who had openly battled drug abuse and alcoholism and was accused by ex-wife Maeve Quinlan of spousal abuse in 1997, suddenly made headlines when he was charged with assaulting an ex-girlfriend. In late 2003 Sizemore, who was required to enter a drug rehabilitation program during his trial, was convicted of six counts and one lesser offense within a count on which he was otherwise acquitted and sentenced to six months in jail and three years probation for abusing his former fiance, former Hollywood madam Heidi Fleiss. That same year he was also sued for sexual harassment by a former personal assistant. In 2005 the actor was sentenced to 17 months in prison and four months in rehab for violating his parole conditions on March 25. He was back in court again for allegedly trying to fake a drug test, not reporting to his probation officer, missing counseling sessions and not advising authorities of his new address. In early 2006 Sizemore violated his probation a second time by testing positive for drugs and getting expelled from a drug-treatment program for fighting, and was sentenced to three years' probation and 90 days in lock-down rehab.