Johnny Depp is generally not known as a method actor, as his recent performances have typically consisted of silly voices and confusing makeup. That said, Depp spent years painstakingly creating his rendition of Hunter S. Thompson for Fear And Loathing In Las Vegas. Depp took to living in Thompson's basement, digging through his manuscripts, interrogating him about his life, and recording those conversations, presumably for personal reference and a sense that the recordings would later be subpoenaed as evidence.
Also, not a scene from the film.
Depp slept and smoked next to barrels of gunpowder that Thompson kept in his house for some unspecified reason (furthermore, Thompson didn't actually tell Depp about the gunpowder until well after Depp should've accidentally blown up the house with an errant cigarette ash). They woke up every day at 9 p.m. and went to bed at 5 in the afternoon. Depp even let Thompson shave his head, after Thompson decided that the haircut the film's costume department had given him wasn't authentic enough.
After living in Thompson's basement for an undisclosed amount of time, Depp took Thompson's red convertible and drove to Las Vegas to meet with director Terry Gilliam, wearing Thompson's clothes that hadn't been washed in 30 years. And he may or may not have done a bunch of drugs. Depp doesn't actually admit to consuming all of the insanity that his portrayal of Thompson does in the film, but he claims to have been true to the character while remaining "very responsible."
"Don't worry; it's not real acid ... it's liquid peyote."