What constitutes a fair Interview?

The police must not abuse their position of trust by conducting an interview in an “oppressive” manner (Code C para 11.3).
Oppression is partially defined by s76(8) PACE 1984 to mean torture, inhuman or degrading treatment and the use or threat of violence. And under s76 (2) (a) can be excluded.

Inducement to confess: Code C para 11.5 makes it clear no interviewer shall indicate an action the police will take if the person makes a statement or refuses to do either.

What are the risks associated with remaining silent?
As suspect has a right to silence and can not be compelled to answer questions. However if he is charged and then advances a defence at trial, there is a danger the court may draw an adverse inference against his silence under s34 CJPOA 1994. As well as the suspect failing to account for objects found on his person (s36 CJPOA) or failing to account for his presence at a particular place (s37 CJPOA).

Unlawfully obtained evidence: in a ‘questionable’ manner (including: confessions) are at a courts discretion to exclude both at common law and under s78 PACE 1984. Common law: provided the evidence is relevant and reliable its admission is not generally regarded as prejudicing the defendant’s right to a fair trial.
S18 allows the court the power to exclude evidence where it would have an adverse effect ion the fairness of proceedings.

Confession Evidence: Where a suspect makes a statement of an incriminating nature in interview, the interview is said to constitute a confession. S82 (1) PACE 1984 defines a confession.
Due to the pressure of an interview it’s possible for a suspect to confess for reasons other than the truth. For this reason the law makes specific provisions for the admissibility of confession evidence. The main provision is contained in s76 PACE and furthermore the admissibility of a confession can also be challenged under s78 PACE if its admission would have an adverse effect on the fairness of proceedings.