According to former New York homicide prosecutor Paul Callan, the referenced senior FBI special agent's ill feelings toward Trump might have impacted his investigative conduct. Callan references the agent's alleged role in changing the former FBI director's conclusion that Hillary Clinton was "extremely careless" with her e-mail oversight vs. the original wording that she was "gross(ly) negligent." Callan added that, "If (the FBI agent) were subpoenaed to testify today before a jury in an ordinary criminal case, his actions would doom the case. Objective jurors would be stunned by such an obviously biased investigator being employed by the FBI in such a lofty investigative position." It remains troubling that certain members of Congress opted to applaud the performance of this agent.
A colleague described this agent's performance as "worthy of a Shakespearean actor or sociopath." He stated that only the best of actors or a sociopath could convince people that their feelings and beliefs would not impact their performance. Which is what this agent did. And then he attempted to corroborate his assertion by referencing the inspector general's report, and said it didn't prove his explicit bias impacted his investigative performance. What he didn't volunteer for context is that there are numerous ways an investigator can impact an investigation that would go undetected.
For example, the decision as to when to interview a subject can impact the outcome. If you interview a high-profile subject on the Friday of a three-day weekend, there is less of a chance of media interest.
Ultimately, how an interview is planned and executed will also impact the outcome. Unless an expert were observing the interview from a closed-circuit monitor, it would be difficult for them after the fact to ascertain the impact of an interviewer's alleged bias.
If an investigator conducts what I call an "apologist interview," the advantage shifts to the subject. If an investigator intentionally leads the subject with soft questions, ignores follow-up opportunities, establishes no rhythm to the question flow, interrupts the subject when they're on the verge of lying, exudes weak body language and an uncertain tone of voice, or delivers questions in a robotic scripted manner, they're discreetly titling the advantage to the subject. Again, it's difficult to assess this unless you're present for the interview.