Here is an excerpt from EW.com’s interview with Chris Martin and Jonny Buckland:

And you also hired a hypnotist during the sessions? What was that about?
BUCKLAND: [Producer] Brian Eno knew a hypnotist and we thought it might get some interesting results. We all went upstairs, sat down, and he walked around us. He got us into some strange kind of trance, and we came down and played some more.
MARTIN: Quite a long time, I think. It did work, actually. We came up with a lot of interesting noises, which we used.

See “Coldplay Talk ‘Viva la Vida’” for the complete interview.

Coldplaying.com posted this quote from Q Magazine:

Another thing that helped them out of the rut was hypnosis, with the band’s new found adventurousness extending to writing music while in an altered state. Some might conclude they’ve misplaced their marbles, but Martin seems unconcerned. “Everything over this past few months has been about taking off any shackles,” he says. “We feel like we have so much to prove and so many ideas that we’d like to try – sometimes you need a hypnotist to give you the bravery to do it.” For the record, he insists the experience, “was fun and interesting, and we wrote some nice things.” Click here for post.