fieldToInterp for a top level component

<component name="PosterWithFadeIn" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" extends="Poster" xsi:noNamespaceSchemaLocation="http://rokudev.roku.com/rokudev/schema/RokuSceneGraph.xsd">

  <children>

    <Poster id="myPoster" loadheight="250" loadwidth="400" width="400" height="250" uri="" opacity="0.0" />

    <Animation
      id = "fadeIn"
      duration = "1"
      easeFunction = "linear" >

      <FloatFieldInterpolator
        key = "[ 0.0, 1.0 ]"
        keyValue = "[ 0.0, 1.0 ]"
        fieldToInterp = "myPoster.opacity" />

    </Animation>
  </children>

</component>

...

the in brs:

m.poster = m.top.findnode("MyPoster")
m.animation = m.top.findnode("fadeIn")
m.poster.uri = < uri >

then during times when you want the animation and the poster to show use this:
m.animation.control = "stop"
m.animation.control = "start"

stop it always before you start it. you can only start a stopped task.