You don't need to lie, but you need to be careful not to frame your realistic skepticism as bias. Then, if you do make the jury, you need to be incredibly tactful so that the prosecutor doesn't get the trial thrown out on the basis that, e.g., you're biased simply because you don't take police testimonies as ground truth.

The risk here is that there are people fighting to get out of prison for things they're not guilty of. If you have realistic expectations that police and their representative prosecutors can and do lie, please exercise caution.