Movies and television shows that have courtroom drama typically feature scenarios where a witness is caught lying and admits his dishonesty to everyone's shock and awe. The reality isn't often quite so sensational. The method of trapping liar witnesses and revealing their deception to the court is called witness impeachment. Impeaching a witness is an endeavour you can refine through practice and study of basic trial techniques.
Collect and study all the evidence that you can get your hands. Such evidence includes statements made by the witness, police reports, affidavits and conversations between the witness and other witnesses.
Take full advantage of the available modes of discovery. Take depositions of witnesses for the opposing side, or serve them with written interrogatories. By doing so, you arm yourself with written or recorded statements that you can use later to catch a lying witness. When taking a pretrial deposition, let the witness ramble on. The more a witness talks, the greater his chances of saying something inconsistent with something he claims later during the trial.
Prepare a summary of the statements that the witness has made on material points of the case for your reference. Note any inconsistencies already made between different documents or statements prepared from the same witness. Identify these as weak points in the witness' testimony. Prepare your examination questions with these statements as foundations.
Lay the foundation for the impeachment of the witness. Ask the witness about the statements he made previous to the trial, especially the ones under oath, such as during the deposition. Ask directly if he made these statements, not if he remembers making the same, since you do not want to create an out for the witness by way of a feigned memory lapse. Quote the statements verbatim for the court to hear, since you want it clear that these are the witness' own words. Throughout, use leading questions -- questions answerable by "yes" or "no" -- to establish your point. Do not give the witness the opportunity to explain anything.
Confront the witness with the lie; compare a statement he made in court with one that you established he made during a deposition or some other prior situation. Do not harp on the contradiction if the witness flatly denies that there is one, or attempts to retract. It is sufficient that you have revealed the error to the court. Do not give the witness the opportunity to explain, either. Move to the next inconsistency swiftly and efficiently, shattering each inconsistent statement in turn.
Question the credentials of the witness when dealing with an expert witness. Do your research, though. Make sure his credentials are lacking. For instance, ask if he has a master's degree or a Ph.D. in the field of his supposed expertise. Ask how many years he has been practicing or studying his field, or how much time he spends weekly on the development of his knowledge and expertise. Do not directly attack what the expert knows, as he is probably much more knowledgeable about the relevant field than you are from just a few weeks of reading. However, do read up beforehand so that at least you will be familiar with the discussion. As before, you can also use prior inconsistent statements to impeach testimony.
Do not be antagonistic or hostile to an opposing witness. If you come across as amiable, trustworthy and pleasant, the witness will be more likely to open up to you and cooperate with you. If you are dealing with expert testimony, hire or consult an expert of your own to help you navigate the expert witness' statements and comb them for possible errors.