First of all, The Revenant is not yet released out in theatres or didn't premiere in a film festival. Then how did Leonardo DiCaprio get nominated for a BAFTA Award for his role in The Revenant?
This document has the rules for the 2016 BAFTAs, including section C. Eligibility, which states:
Films must open for theatrical general release for the first time in commercial cinemas in the UK between 1 January 2015 and 12 February 2016
- Films which open after 1 January 2016 must be qualified by being screened to BAFTA members (‘voters’) no later than 17 December 2015; otherwise films opening after 1 January 2016 should be entered into the 2017 Awards.
The UK release date for The Revenant is currently 15 January 2016 so within the deadline of 12 February 2016. There is more than one article online indicating that it's already been screened for "awards voters" in November, so that's also within the 17 December 2015 cutoff.
The screening for the movie took place in November where members of the press and award voters were present. The screening was held in New York. They were impressed with his performance and so he got nominated.
For more details see this Vanity Fair article.