Hola Gramatica!
A mi entendimiento.......
-Dijo que era muy interesante- leaves a lot of room for deciding whether you will be using (era) as an imperfect verb or a preterite....
-Dijo que el programa
era muy interesante.-
-Dijo que el participar en el programa a diario
fué muy interesante.-
To translate what you wrote, I would say.....
-Dijo que era muy interesante = He said it was really interesting.
-Dijo que fué muy interesante = He said that it was really interesting.
You know that using a preterite = a single action whereas using the imperfect would mean = a repetetive action or something done more than once.
I used to get very confused until I engrained the above rule in my head!
Elaina
