Drake Bell Details Molestation by Brian Peck in Nickelodeon Documentary

Drake Bell is finally telling his story on how Brian Peck molested him as a teen -- detailing the case at the time ... a period during which celebs came forward to support his abuser.

The new ID doc 'Quiet on Set' had its media embargo lifted, and people who've seen an early screener -- including us -- are detailing what exactly Drake says here ... and it's pretty hard to watch.

Drake gives the blow-by-blow of what happened between him and his former dialogue coach -- saying Brian drove a wedge between him and his father back then, who was working as Drake's manager ... and who was apparently suspicious of Brian.

In terms of how the abuse started, Drake says Brian isolated him a lot while working on Nickelodeon shows -- and he'd often end up hanging out with Brian outside of the set including at Brian's home, and had spent the night there many times. One morning after sleeping on his couch, Drake says he woke up to find Brian sexually assaulting him.

From there, Drake says things got worse ... and the abuse became more repetitive -- and he says he felt trapped by a dark secret. Eventually, he broke down and confessed to his mom. Once his mother became aware, they got cops involved -- and Drake says he told them everything. He also claims he got Brian to confess to the police on a recorded phone call.

Eventually, Brian was arrested on multiple counts ... but he pled no contest to just two -- performing a lewd act with a 14 or 15-year-old and oral copulation with a minor under 16.

One of the more startling revelations in the doc is that as Brian was set to be sentenced, he had a crap ton of famous people and friends show up on his behalf in court ... and not just that, but several stars had written character letters on his behalf, including some big names.

James Marsden wrote one, as did Taran Killam ... not to mention "Boy Meets World" stars Rider Strong and Will Friedle -- who've already expressed regret over this in recent weeks.

"Growing Pains" star Joanna Kerns also wrote a letter for Brian, but came forward in the doc to say she only did so based on misinformation. The late Alan Thicke also wrote a letter in support ... and so did "The Suite Life of Zack & Cody" directors Rich and Beth Correll.

Peck briefly went on to work on the 'Suite Life' after he served his 16-month prison sentence post-conviction. We've reached out to Marsden, Killam and the Corrells ... no word back yet.