Patty Duke, the ex-child star who brilliantly portrayed Helen Keller in The Miracle Worker, today lives in rural Idaho and earns a solid living from television movies and the occasional Broadway show. What would motivate her to come to the Chicago suburbs to perform at a small nonprofit venue? "When you have the opportunity to interpret really wonderful writing, many of us journeymen actors will go anywhere," says Duke of her upcoming engagement in A.R. Gurney's Love Letters at the Theatre of Western Springs. Just as important, she adds, is the opportunity to costar with longtime friend John Grant-Phillips, the venerable theater's new artistic director. Gurney's work charts the 50-year relationship between a straitlaced male lawyer and a rebellious, unstable female artist in letters read by the two actors sitting at tables. It's a device well suited to a performer of Duke's expressiveness, and this is a fine opportunity to see her in an intimate setting. She brings an unusually keen understanding to the troubled woman she portrays: diagnosed with bipolar disorder in 1982, Duke is a national spokeswoman on mental-health issues. "It made me a little self-conscious the first time I played the role," she admits. "Certain passages hit mighty close to home. But I love her in the way I love the part of me that has so many faults. The play's focus is her huge attempt at bravery even though the attempt fails. And her dark sense of humor knocks my socks off." Opens Thu 2/10, 8 PM. Through 2/13: Fri 8 PM, Sat 2:30 and 8 PM, Sun 2:30 PM. Theatre of Western Springs, 4384 Hampton, Western Springs, 708-246-3380. $55; additional $25 for reception after each performance.