It is for the jury, who have seen and heard the witnesses, to decide whether they accept their evidence. They are free to accept or reject the whole of a witness’s evidence, or to accept some of the evidence and reject the rest (Cubillo v Commonwealth (2000) 174 ALR 97; Flint v Lowe (1995) 22 MVR 1; S v M (1984) 36 SASR 316).
It is therefore customary to direct a jury that they are not bound to believe the evidence of any witness, and that they are not bound to believe the whole of the evidence of any witness. They should be told that they may accept some parts of a witness’s evidence, but not other parts (Cubillo v Commonwealth 174 ALR 97; Dublin, Wicklow & Wexford Railway Co v Slattery (1878) 3 App Cas 1155).
The jury should also be directed that it is their duty to keep an open mind about the truthfulness of any individual witness, and about the accuracy of that witness’s recollection, until all the evidence has been presented. It is only once they have heard all of the evidence that it will be possible for them to assess to what extent, if any, that witness's evidence has been confirmed, explained or contradicted by the evidence of other witnesses. Only then should they direct their minds to the question of whether the guilt of the accused has been proved beyond reasonable doubt (Haw Tua Tau v Public Prosecutor  AC 136).
Where a witness was intoxicated at the time of the event about which he or she is giving evidence, it may be appropriate to direct the jury that the reliability of his or her evidence may be affected (see, e.g., O’Leary v Daire (1984) 13 A Crim R 404; Bedi v R (1993) 61 SASR 269; R v Mathe  VSCA 165; R v Baltensberger (2004) 90 SASR 129; R v MC  VSCA 122).
The judge, prosecutor or defence counsel must not suggest in any way to the jury that an interest in the outcome of the trial is a factor to take into account in assessing the evidence of witnesses generally (Jury Directions Act 2015 s44H, as amended in 2017).
This is because the jury will likely conclude that the accused has the greatest interest, and so the direction may have the effect of undermining the presumption of innocence (Robinson v R (No 2) (1991) 180 CLR 531; R v McMahon (2004) 8 VR 101; Hargraves & Stoten v R (2011) 245 CLR 257; Department of Justice and Regulation, Jury Directions: A Jury-Centric Approach Part 2, 2017, 11-14).
Suggestions that a witness has a particular interest in the outcome of a trial are permitted. For information on directions about assessing the evidence of an accused, see The Accused as a Witness.
It is undesirable to suggest that a conviction or acquittal will reflect favourably or unfavourably on the credit of a witness, or have some other effect on them (R v Coulston  2 VR 446).
The jury should not be told that witnesses must be presumed to be innocent. The presumption of innocence is only relevant to the accused (Howe v R (1980) 32 ALR 478).
 See Common Law Intoxication for information about other directions that may be necessary in cases involving intoxication.