How Crazy Thinks

What Fincher's Mindhunter gets right about user research.

EssaySeptember 20244 min read

In the late 1970s, two FBI agents started interviewing convicted serial killers. Holden Ford and Bill Tench, as depicted in David Fincher’s Mindhunter, sat across from Ed Kemper, Charles Manson, and others. Not to get confessions. To understand how they think.

Bill Tench in the Behavioral Science Unit office — Bill Tench, BSU

Criminal profiling and user research solve the same problem: people don’t tell you the truth about their own behavior. Not because they’re dishonest, but because memory is unreliable, ego is protective, and most people can’t articulate why they do what they do.

Holden Ford, Dr. Wendy Carr and Bill Tench together in the BSU basement office — The BSU team

Analytics tell you 40% of users drop off at checkout step three. A conversation reveals they leave because the shipping cost felt like a bait-and-switch. Ford could have read case files. None of that would have given him what he got from sitting three feet from Ed Kemper.

Ed Kemper seated in the prison interview room — Ed Kemper

The interview

Empathetic detachment

Ford doesn’t jump into the crimes. He starts with small talk: prison life, interests, daily routine. When someone feels comfortable, they share real frustrations instead of performing for you.

Ed Kemper seated across the interview table from Agent Ford — Kemper across the table

FBI agents interview killers without moral judgment. User researchers need the same detachment. When someone criticizes your design, the urge to defend it is strong. That’s your cue to lean in. They’re revealing how they actually think about your product.

In interviews on a savings app, users kept mentioning raffles and lotteries as a factor in choosing where to invest. We hadn’t asked about it. It kept coming up unprompted. We built a feature that let users forego interest for raffle entry. That insight came from listening without steering.

Start with mundane questions. Use their words, not your jargon. Follow their detours before redirecting. The insights live there.

Scripts vs. spontaneity

A script gives you consistency. Rigid adherence kills spontaneity. When someone mentions something unexpected, follow that thread.

Raw interview transcripts from the savings app research, coded by theme — Savings app transcripts, coded

On a savings app, a user mentioned timing deposits on payday helped them save more. This wasn’t in my questionnaire. We followed it. That insight shaped the entire product.

The shipped deposit timing feature in the savings app — The deposit timing feature

Asking the right questions

“Do you like our checkout process?” invites performance. “Walk me through the last time you almost didn’t complete a purchase online” invites memory.

Super users

Kemper became the BSU’s most valuable informant. Dr. Bryanna Fox said he cooperated because being consulted by the FBI made him feel special. Every researcher has encountered this: the super user who’s deeply engaged. Cultivate them. But don’t over-index. Kemper was still one data point.

From anecdotes to patterns

The real work happens after the recorder is off.

The BSU team reviewing interview material back at the office — Analysis after the interviews

The BSU organized patterns into a taxonomy: organized vs. disorganized offenders. Organized offenders were intelligent, methodical, brought tools to the scene, left little evidence. Disorganized offenders were spontaneous, used whatever was at hand, left evidence everywhere.

Crime scene characteristics told investigators what kind of person to look for. You do the same: review transcripts, tag recurring themes, group them into categories that predict behavior.

Interview highlights coded by category in Dovetail — Thematic analysis in Dovetail

On a credit card project, the industry splits users into “transactors” (clear their balance) and “revolvers” (carry debt). But several transactors mentioned they’d been revolvers in college. The categories weren’t fixed identities. They were life-stage behaviors.

We also found security-conscious users who routed purchases through PayPal, feature-comparers who analyzed five banks before committing, and status-signalers who cared about card design as much as interest rates. One said “I want one of those Monzo orange cards!” None of these map to demographics. They predict product interaction.

Flowchart contrasting link-dominant users, who browse the aisles, with search-dominant users, who ask the clerk — Link vs search dominant, after Krug

Where interviews mislead you

Social desirability

People want to look good. “I love this feature.” Their usage data says they’ve never touched it. Ask about past behavior, not hypotheticals.

Post-hoc rationalization

In credit card interviews, every participant claimed to know their late payment interest rate. When we checked, they consistently underestimated it. They’d constructed a version of their financial literacy that felt true. Focus on what they did, not what they say they did.

Rushed context

The most revealing interviews in Mindhunter aren’t the first ones. They’re after multiple visits, when trust exists. For us: diary studies, repeated interviews, ongoing relationships.

Ford and Tench conducting a later interview, after several visits — A later interview