Despite increasing uptake of bystander intervention programs to prevent sexual aggression, rates of sexual violence have remained persistently high. Those who witness sexual aggression among their peers can provide another vantage point regarding the strategies that perpetrators use and valuable information about ways in which perpetrators divert bystanders’ intervention—all information that can inform prevention programs. Participants (N = 247) completed structured and open-ended items about occasions they had witnessed that involved efforts to force sex on a non-consenting individual. Reports were content coded for strategies leading to sexual aggression for 99 participants who had witnessed a recent alleged act of sexual aggression. Most (93%) reported perpetrators’ use of early physical pressure (e.g., unwanted grinding, following, isolating, violating personal space, pulling, blocking others) that typically escalated into more overt physical pressure and force. Verbal coercion (e.g., arguing, insisting, begging) was witnessed by 40% of participants, and 14% of participants reported witnessing the target being pressured to consume excessive levels of alcohol. Coded themes captured perpetrators’ defensive interactions with concerned bystanders, such as making excuses, minimizing their intentions, feigning innocence, and using humour to divert attention from sexually aggressive efforts. Results have implications for prevention efforts incorporating bystanders as well as education about the risk of assault.