The overall key to a successful police interrogation is patience and the ability to communicate well during the interrogation. Apply the tactics and skills you have learned to a real interrogation.
Patience and communication are both valuable tools for interrogation. There are other important skills needed to encourage an admission and then a confession.
When starting an interrogation it is important to build rapport, a bond between yourself and the person being interrogated. This can be done by identifying common backgrounds, styles, even commenting on a tattoo. People generally like to talk about themselves. Focusing the beginning questions on who the person is will allow you to gain knowledge about how they react and when they are telling the truth.
Moving into the interrogation questions should be gentle. This means the questions need to be open-ended and assume the person has knowledge of the subject. For example don't ask "Were you at the party?"; this is a closed questions (requires a yes or no answer) and gives the person the ability to say no. Instead use, "Tell me about the party"; this allows the person to deny they were at the party, but also encourages them to tell you about the party because there is an automatic assumption you know they were there.
Let the person talk; this is where patience comes into play. The subject may inadvertently say something useful to the investigation during a rambling answer to a simple question. You may have to ask the same question multiple times in different ways.
Toward the end of the interrogation you'll need to develop themes and present those to the subject. Themes are meant to elicit an admission. An admission is accepting responsibility for part of the crime, but it is not a full confession, which is a description of the entire event. Themes allow people to accept responsibility gradually. You don't need to believe the theme; it should just be believable to the subject.
"It's not your fault; I know you took the money from the party but there was drinking going on and you have bills to pay. I mean, pawning the watch can help you feed your family. You were just trying to feed your family, who doesn't understand that?"
"I know you took the watch, but you were dared to do it. Who hasn't been dared to do something? You gotta stand up for yourself and I'm sure you were going to give it back, you didn't really mean to steal it...it was more of a dare that got out of hand."
As soon as you deliver the theme, you have to give the choice statement. This is a choice between accepting the theme or accepting a more heinous accusation.
"Did you steal the watch for yourself, or did you steal it to feed your family?"
"Did you steal the watch because you're a thief, or did you take it on a dare with the intention of returning it?"
Once the person admits to the theft, the interrogation begins again. Now you have to build some rapport, let them relax and grow accustomed to the admittance. Then you begin to get the details.