Obviously, some form of attraction to people of different genders is required, but the exact extent varies for some people, an offhand comment or a prolonged glance can be enough for others, nothing short of onscreen sex can suggest bisexuality as a possibility.
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The character does not have to officially be declared bi to qualify, and as No Bisexuals demonstrates, even if a character is clearly bisexual to the audience, much of the time the writers dont consider it as a possibility. It should be noted that what makes a character Ambiguously Bi and what differentiates them from Bisexual characters is largely a matter of opinion. Getting Crap Past the Radar can be a reason for this trope, the writers wanting to include hints a character is gay or bi but with enough heterosexuality to appease the Moral Guardians. Characters with a Heterosexual Life Partner are especially prone to this, as while they might have an actual love interest of a different gender or try to get a love interest through flirting and having crushes on women, the relationship with their HLP often has a romantic/sexual element as well. Most of the time, this character's interest in people of a different gender is explicitly stated or written to be obvious, while their interest in people of the same gender is subtle or downplayed, but the inverse does exist.