Challenge the Witness's Credentials and Professional Background
Your attorney may seek to impeach the witness's credentials, qualifications, ability to know or bias against your client (or DUI defendants in general). Depending on the witness being offered by the Government, this can be VERY effective. If the witness is well-qualified, the cross-examination may only focus on an issue such as BIAS. For example, "Mr. Wilson, isn't it true that you have never been employed by any private law form or laboratory, and have only worked for one forensic science employer since you left college?"
Impeachment of the Direct Testimony of a State Witness is Likely the Most Common Cross-Examination actic.
The most common attack on cross-examination is to Impeach the TESTIMONY of the Government witness. A well-prepared criminal defense lawyer knows that to do this, he or she needs to have "ammunition" to use during cross-examination. If the Witness has either written some article or manual or letter that argued or posited a CONTRARY opinion, or has testified under oath in a court of law or deposition for another civil or criminal case to the CONTRARY, then the witness can be shown to have previously given other sworn or published opinion, which can destroy his or her credibility in the current case. Your lawyer may "set up" the State's expert by getting him or her to concede certain "alternative explanations" or theories, and then (during the Defendant's presentation of evidence) put up eyewitnesses or medical witnesses that clearly prove that the State expert was 180 degrees WRONG in his or her testimony. This IMPEACHES the State witness, to show the testimony is unworthy of belief.
Cross-Examination can be Used to "Make the State Witness YOUR Witness" and Advance Your Theory of a Flaw in the Testing Procedures
Your legal counsel may choose to not attack the witness as shown in the two previous sections, but may opt to use cross-examination to advance the DEFENSE THEORY of the case. This method of cross-examination often utilizes the published articles or studies of a world-renowned expert in the field of breath testing, blood testing, laboratory quality controls, electrical engineering or infrared spectroscopy. For example, assume that your attorney possesses a jail video of your breath testing that shows the police officer using his cell phone to make and receive calls during your breath testing. Studies and experiments on breath testing devices have shown that RFI (radio frequency interference) can case FALSE POSITIVE readings on breath testing devices. This means that radio waves can be falsely reported as BREATH ALCOHOL. I have attached two very important links below to demonstrate this type of error during experiments with breath devices.
Where the Witness Used by the State Offered Opinions that are NOT Backed Up by Factual Evidence, Cross-Examination can Unravel Such Evidence
In many DUI-DWI cases, police officers gather only PART of the critical evidence from an accident location and from the suspected DUI driver. By getting the State witness to concede how many ASSUMPTIONS he or she made in telling the jury what the Prosecutor wanted them to hear, your Attorney may be able to demolish the State's theory of the case. Example: An officer comes to an accident location, where a car has struck a telephone pole. The officer smells alcohol on the driver's breath, and asks if she has been drinking. She answers, "Yes, but I only had two small glasses of wine at home." The officer arrests her & later collects a blood test that shows a 0.049 grams per cent alcohol content. If the State expert tries to claim that the driver's alcohol content AT THE TIME OF THE ACCIDENT 2 hours before the blood was drawn WOULD HAVE BEEN HIGHER, due to elimination, the Cross-examination would ask: "How do you know her alcohol level was not RISING?" (Studies show that this is possible)