A significant interaction is very unlikely if you are only now just thinking about trying methadone (since i assume the dose will be low, probably under 40mg unless you are tolerant to opioids).
The concern with respect to methadone and serotonergic antidepressants generally takes two forms: the potential for serotonergic crisis (methadone is somewhat serotonergic, particularly at high doses) and/or increased risk of cardiac arrhythmia. Generally these are dose dependent or seen in the predisposed (the elderly, people with undiagnosed heart conditions, etc).
10mg of lexapro with say 10-20mg of methadone should produce no serious interaction in a healthy individual who has no preexisting condition. The biggest risk you have here is simply the very act of taking methadone, which is easily the most dangerous commonly abused pharmaceutical opioid in terms of accidental overdose deaths (in proportion to the frequency with which it is abused).
A big part of this stems from a simple fact: most people who are maintained on methadone take a daily dose that would be either fatal or nearly fatal to a non-tolerant adult. If that gets in the wrong hands it is poison. Though I've been long off it, I once was on 380mg of methadone/day. If I took that now I would without question die within a few hours (in the absence of medical attention).
In any event your concern shouldn't really be around the interaction (unless predisposed) but more about the danger of methadone itself. For instance, what dose to take, or even, how to determine the dose since methadone often changes hands in the form of a liquid of unknown concentration).
