Haas made his feature debut playing the youngest child of Jane Alexander in "Testament" (1983), a harrowing and realistic drama about the aftermath of nuclear war. "Lady in White" (1988), a minor genre classic, provided Haas with his first starring role: an inquisitive and imaginative boy confronted by both the supernatural and a serial child killer. Brit culture magazine TIME OUT deemed his performance "luminous". Haas was also impressive in "The Wizard of Loneliness" (also 1988), a WWII-era drama about a city boy sent to the country to live with his eccentric grandfather. He moved confidently into older teenage roles, particularly as an adolescent who receives an incidental introduction to the female anatomy from the title character (Laura Dern) in "Rambling Rose" (1991).
Haas also did substantial TV work from the mid-80s through the early 90s. He brought a palpable sense of wonder and enthusiasm to the Steven Spielberg-directed "Ghost Train" (1985), the lavish debut of the fantasy anthology series "Amazing Stories". Haas was effectively unsettling as a boy's "imaginary" friend in a 1986 episode of the CBS revival of "The Twilight Zone". He appeared in some half dozen TV-movies, receiving the most attention for his poignant starring performance as the brave 13-year-old hemophiliac battling AIDS and prejudice in "The Ryan White Story" (ABC, 1989). In 1998, he played the troubled David in Oprah Winfrey's TV movie remake of the touching "David and Lisa" (ABC).
The early 90s proved an awkward period for the growing actor. Having moved with his family from Los Angeles to Austin, Texas several years before, Haas spent most of his high school years out of the spotlight. He returned to the screen in 1996 with roles in three features: "Boys", as a prep school senior who rescues and becomes smitten by an "older woman" (Winona Ryder); Woody Allen's period musical comedy "Everyone Says I Love You" and Tim Burton's high profile sci-fi epic "Mars Attacks!". Haas followed these performances with a small role in Terence Malick's star-studded 1998 adaptation of the war drama "The Thin Red Line".