There are 2 types of criminal matters which appear before the criminal courts in Thailand. The first would be a case where the public prosecutor has accused the defendant of criminal activity. The second type of case is one where it arises from the civil lawsuit.
In terms of Section 39 of the Thai Constitution B.E.2550 (2007), the defendant is innocent until proven guilty. The first part after an arrest is that you should have the ability to apply for provisional release. In the West this would be called applying for bail. Section 108/1 of the Criminal Procedure Code there are limitations on bail or provisional release. Under this provision the courts can deny bail if they believe that the defendant will leave the country or tamper with evidence or cause other difficulties.
On request for provisional release the court will either decide to allow or not allow the release. In terms of Rule 8 of the Official Practice for Judicial Judges governing Provisional Release of 2005 the court may also place conditions to the provisional release such as report to court at a certain time. This would also be the same in the West where you may be required to report to the police once a week.
With foreigners the court would have to ensure that the person cannot leave the country. The court may decide to hold the persons passport but this comes down to the court itself. Much the decision lays in what the foreigner has been accused of. Drugs would be a major crime especially trafficking. Getting bail for those types of crimes would be very difficult. Normally the court would want your passport as well as money large enough to ensure that you return to court.
The court order will be sent to Thai immigration to ensure that you cannot leave. If however you are a businessman you might be allowed to leave the country to continue doing business while the court proceeds. This is to ensure that when you need to leave that each time you leave the country you don’t need the court to give you permission to leave. This would make business near impossible to do.
The criminal courts in Thailand would need the defendant to be in court during the trial. However, the trial can be conducted in the absence of the defendant in court. The legal process in Thailand is conducted in the Thai language and as a foreigner their presence in court would not normally be needed as they will not understand the process or the arguments. The necessity to be in court would thus be minimal unless the defendant’s presence in court is ordered by the judged. Without the court’s order, the presence of your attorney will be enough to ensure that you have received a fair trial.
Never sign any documents which may be handed to you after you have been arrested. This will normally be in the Thai language and you will need to have legal representation to explain to you what is being signed. It is very costly and difficult to correct these signed agreements later as it might be an admission of guilt or something else. You do have the right to legal counsel so ensure that you make use of that right.