Working alone as a graphic artist, Hartman frequently amused himself with "flights of voice fantasies". Citing the need for a more social outlet for his talents, Hartman, aged 27, began in 1975 to attend evening comedy classes run by the California-based improvisational comedy group The Groundlings. While watching one of the troupe's performances, Hartman impulsively decided to climb on stage and join the cast. After several years of training, paying his way by re-designing the group's logo and merchandise, Hartman formally joined the cast of The Groundlings; by 1979 he had become one of the show's stars.  Hartman met comedian Paul Reubens and the two became friends, often collaborating on writing and comedic material. Together they created the character Pee-wee Herman and developed The Pee-wee Herman Show, a stage performance which also aired on HBO in 1981. Hartman played Captain Carl on The Pee-wee Herman Show and returned in the role for the children's show Pee-wee's Playhouse. Reubens and Hartman made cameos in the 1980 film Cheech & Chong's Next Movie. Hartman co-wrote the script of the 1985 feature film Pee-wee's Big Adventure and had a cameo role as a reporter in the film. Although he had considered quitting acting at the age of 36 due to limited opportunities, the success of Pee-wee's Big Adventure brought new possibilities and changed his mind. After a creative falling-out with Reubens, Hartman left the Pee-Wee Herman project to pursue other roles.  In addition to his work with Reubens, Hartman recorded a number of voice-over roles. These included appearances on The Smurfs, Challenge of the GoBots, The 13 Ghosts of Scooby-Doo, and voicing characters Henry Mitchell and George Wilson on Dennis the Menace. Additionally Hartman developed a strong persona providing voice-overs for advertisements.

Answer this question "What were the possibilities?" by extracting the answer from the text above.
After a creative falling-out with Reubens, Hartman left the Pee-Wee Herman project to pursue other roles.