Says Peter Linz, the puppeteer who won the role of Walter, says that while he was auditioning, he asked what he should be looking at for inspiration. Linz says he was told to think of SUPERBAD star Michael Cera. Said producers, “If [Cera] was a puppeteer he would already have the job.”
As it turns out, Cera was considered to voice Walter but in the end, Segel and co-producer Nicholas Stoller wanted to avoid using celebrity voices so as to not distract people. You may remember that Paul Rudd was also rumored to be voicing Walter but Segel says that was never the case. Instead Rudd was in talks to play a human version of Walter during a dream sequence but the scheduling never worked out.
Without having seen the film, I can’t say how much Walter feels like a Michael Cera creation but it’s always interesting to see where the puppeteers get their inspiration for their characters.