By Stephen Saito
In a festival that's boasted such fine music docs as "Lou Reed's Berlin" and "Playing for Change: Peace Through Music," along with an appearance from Madonna to promote the non-musical Malawi doc "I Am Because We Are," Tribeca has also turned out to be a place where musicians put down their instruments and pick up scripts. Though acting is nothing particularly new for either Mariah Carey or Dave Matthews, the two have taken on supporting roles in the low-budget films "Tennessee" and "Lake City," respectively, both in this year's line-up. Here's a look at how they measured up.