With this expression, the text in the marker will be duplicated in the text layer. Thus, on a single text layer, you can use many different words, without using the animation keys in the Source Text parameter. The expression must be pasted into the Source Text parameter of the text layer.

m = thisComp.layer("Text Source");
n=0;
if (m.marker.numKeys > 0){
n = m.marker.nearestKey(time).index;
if (m.marker.key(n).time > time) n--;
}
if (n > 0) m.marker.key(n).comment else  ""