Your question is whether you can directly contact a plaintiff in your lawsuit. The answer depends on whether that party is represented by an Attorney.
If someone is represented by counsel it is unethical for another Attorney to contact them directly. There is not such restriction on a non-Attorney, but the represented party would be smart to let their lawyer do the talking.
Unfortunately, many people who hire Attorneys continue to discuss their case with opponents directly. This kind of defeats the purpose, but it still happens all the time. It is unethical for Attorneys to contact one who is represented; except throught their lawyer. This rule is based on the idea articulated above, as well as by the concern that unrepresented parties not be unduly influenced.
I hope this information has been helpful. Feel free to contact our office with any further questions.