The actor dished Saturday about what exactly went into preparing for the job when he was cast by Ryan Murphy and co. to play the lead in "Dahmer -- Monster: The Jeffrey Dahmer Story" ... and, as it turns out, there was a lot that went into embodying the guy.

During a Q&A, EP revealed he actually wore Dahmer's actual clothing and accessories -- including his glasses, his shoes and his jeans. How exactly the production got their hands on that stuff to begin with is anyone's guess ... but Evan says he put it to good use for months.

He even says he wore weights on his arms to help maintain Dahmer's straight-as-an-arrow posture, and Murphy had previously revealed Evan stayed in character while he was on set.

Luckily, though, the guy didn't take Dahmer home with him ... and had some techniques to shake the darkness he was living with on a daily while shooting the show. Some of that included watching comedies and lighthearted content, not to mention feel-good music.

He specifically cited "Step Brothers" as one flick he'd watch to drop the Dahmer effect when the cameras weren't rolling ... clearly, becoming Dahmer took its toll on the dude.

Evan's costar, Niecy Nash, also spoke her piece at the panel ... and said she was glad the series examined different lenses into the Dahmer story -- especially as it pertains to the victims and his watchful neighbor, Glenda Cleveland, whom NN portrays on the show.

She addressed the racism and homophobia present in the saga -- especially on the part of cops at the time -- and noted they'd done a good job at capturing that facet of it.

There's been outrage over the project being made, as some feel that it glorifies Dahmer. However, those who've actually seen it in its entirety know it's more nuanced than that ... showing this tragedy from multiple sides and not hiding from the truth of what happened.