Trust is the most important ingredient for successful fisting. Without it, the Bottom cannot enjoy the playtime fully due to the inherent dangers of fisting. Likewise, the Top may not achieve full arousal upon brachial penetration if he knows the Bottom is on edge (unless, of course, he's a sadist).
There are several factors that will enhance or diminish trust. Keep in mind the opposite of each enhancer or detractor affects levels of trust in the inverse direction.
Being familiar with your partner increases trust. Existing relationships tend to have the highest levels of trust, but shared commonalities with a new partner can bridge the time-gap need to build familiarity.
Commonalities include items such as similar ages, languages, geographic proximities, religious upbringings, club memberships, and likes/dislikes—as well as shared cultural ties.
Unless monogamous, you and your partner(s) are likely to be adding new ff-Buds all the time. While familiarity may be low with new partner(s), and compatibility cannot be determined completely until after a first session, honesty immediately plays the major role in building trust with new playmates.
Misrepresenting yourself in your profile (by photo or description) creates all kinds of red flags. Not following your word and innumerable excuses also affect trust. Withholding information and lying are deal breakers.
Emotional Quotient (EQ)
Emotional intelligence (EI) or emotional quotient (EQ) is the capacity of individuals to recognize their own, and other people's emotions, to discriminate between different feelings and label them appropriately, to use emotional information to guide thinking and behavior, and to manage and/or adjust emotions to adapt environments or achieve one's goal(s).
Being able to read the Bottom is critical for Tops, and being able to project emotions and approval is critical for Bottoms. There is a high level of non-verbal communication during a fisting session and the capacity to understand and process that information is extremely important.
Skill levels of both partners can affect trust. Inexperienced Tops may not be able to successfully navigate the fist chute while simultaneously reading non-verbal communication and processing other items/distractions that may occur while deep inside the Bottom. Novice Bottoms lack the ability to relax and let go because they don't have knowledge of the process nor an internal roadmap.
Compatibility covers anatomies, personalities, perversions, preferences, and schedules. Most important are anatomies, personalities, and preferences. If these don't match, then it's difficult to build trust and confidence in each other.
As mentioned previously, there is a high level of non-verbal communication taking place during a fisting session. Drugs alter the ability to process that communication effectively.
Arrogance / Narcissism
Egos have no place in fisting.
Depth fisting is a gentle touch and never a rushed activity. Individuals who display overt abrasiveness demonstrate that they do not comprehend fisting practices or demeanor that keeps everybody safe.
There are bad apples out there! References from the fisting community provide insight into who can and cannot be trusted.
Hygiene / Appearance
Fisters are generally the least judgemental people in the world. They maintain a live-and-let-live attitude. However, personal hygiene and unkempt appearances can override that attitude and affect trust. Bad breath, unpleasant body smells, and odors trigger subconscious reactions outside our control. Lack of basic cleanliness makes people question your overall mental abilities.
On occassion, you will find that some fisters are caught up with qualities such as age, weight, fitness, prosperity, and race/ethnicity. Try to avoid such assholes.