Anything and everything that would be appropriate to the circumstances. Statements from the respondent him or herself, challenging the statements provided by the complainant, obtaining witnesses, obtaining witness statements, text messages, social media posts, emails, phone logs, video recordings of whatever may have taken place, surveillance footage, that is, swipe card access for dorm rooms, for example.
It's possible to get expert reports as appropriate to the circumstances, be it a forensic evaluation or psychosexual evaluation. Forensic toxicology reports can also be obtained for many cases, say, that involved consent or intoxication versus incapacitation. Pictures, videos, anything that could possibly help defend against a allegation of sexual misconduct.
There's no overkill in these cases. So much is at stake. You have to both be appropriate, but also creative, in terms of defending against such a case and an experienced attorney advisor can help do so, can help let you know what evidence would be most appropriate, in terms of mounting the strongest possible defense. They should be involved as early as possible in the process.