Sure, you can try a few things.
While you’re creating, remember that YOU ALWAYS KNOW it’s “the same voice”. OTHERS DO NOT!(Necessarilly - don’t give anyone too much credit - ha!)
If it’s OK to sound “cartoony” you have much wider latitude. However, most of the time these are subtle changes, if not enough to fool anyone, at least enough to give them the idea that you’re trying to fool them - and most will automatically “suspend disbelief” and play along, with no problem - just like we all do when we go to a show or a movie.
Stuff to try:
Vary the EQ of the “stern” voice. Add bass/Remove treble(Or the reverse - whatever works.).
Try “Pitch change”(While automatically maintaining timing - or not.). as little as a 1% change gives any voice a whole new sound!((For instance: If you “raise” the pitch of all the other voices, the “normal” voice sounds “stronger”, older, more authoritarian.Though it’s easier just to lower the one voice… You get the idea.))
Reverb or echo on just one voice(Even a tiny bit, un-noticable to most listeners.) can add some “authority” compared to the “bare” voices. Again! OR the reverse! Everyone slightly echo’ed except the “main” voice…?
If one were to do some experimenting, one should be able to create a “suite” of effects/adjustments that would slightly to drastically modify “the same voice” in a wide variety of ways. Make up your effects list and call them voice 1, 2, 3, 4, etc. Or give 'em names! Happy/Sad/Mad/Silly/Dad/Junior, etc.
Cut or extend the time between words or phrases(Timing correction with No Pitch Change can work.), just like real people do, when they all read the same paragraph aloud. Some folks speak faster, some slower - TINY changes!
Again, if being “QUITE DIFFERENT” is allowed - Sometimes the BIG, BASS VOICE on the car dealer commercial is “filtered”(To sound like a telephone voice), for a few words, for “contrast” and “punch”.
What you are looking for is something “different”, but, often, very little different. Just enough to convey an attitude or emotion - stern/happy/sad/scared(Make it “waver”.), etc. - COMPARED to the “other voices”! VO folks do multiple voices in the same project all the time(Saves money for client, makes more money for talent). Almost no one but the producer and engineer ever knows… Not that VO people are always that good, perse’, just that no one is listening for such things and it never occurs to them that the voices talking to each other, which sound slightly different, are the same person. In reality, while Mel Blanc was the master of character voices, even his were not all that different from his general speaking voice(Really hard to hear a recording of Mel “just talking” - how would you know? Ha!) - Close your eyes and “listen” to an old Warner Brothers cartoon and “hear” - without the visual of the cartoon character, it’s “the same voice” just a bit “off”. You can likely create a pseudo(Good enough for the purpose) version, electronically.
Speaking of cartoon voices, don’t eschew them! Use 'em! There are very real people who speak in really weird voices. The really unusual voices are great for a few words in alot of scripts. Or alot of words in a really bad script that needs help(Sometimes, the voice - often a celebrity - is the only good thing in an animated film or “same old, same old” commercial for “name any advertised product”. IOW: We don’t always want people to actually pay attention to the words - ha?).