@SuburbanVagabond Pretend to share their views 100% at first (while being careful not to let their propaganda or even your own words affect your real views, mind you), gain their trust over time as you become a fairly well-known regular in their community, then (after a long enough period... I'd say at least weeks, better months) the payload: start subtly & carefully exploiting the logical inconsistencies in their ideology to seed doubt, discord, & sectarian division w/o raising too much alarm.
@IrelandTorin How does a person make sure they don't like anything back to their usual profiles? Right now I'm using a whole separate browser, email, usernames, generated passswords and the like.
@SuburbanVagabond That's a very good start - I would also recommend using a different IP (proxy, VPN, Tor, I2P, whatever).
Actually, one of the best things you can do: create a completely separate user on your computer for this fictional person. And it's better to use Linux if possible (even better if you don't mention you're doing so) to dodge most malware, as that can actually pose an exposure risk.
May also be a good idea to have a stylesheet / traitlist for this "new person" for reference.