Hilary Duff is FINALLY able to dish on the How I Met Your Father season finale and unveils surprise guests

Hilary Duff shared some behind the scenes photos of How I Met Your Father on Instagram, revealing some fun she had with a couple of surprise guests from the comedy's season finale.

Cobie Smulders - who played Robin Scherbatsky for nine seasons on the award winning How I Met Your Mother - made a cameo in the final episode, offering some advice to Hilary's character Sophie.

The two characters met during a chance encounter in the fictional McLaren's Pub and, surprise, behind the bar was Joe Nieves as Carl.

The news is out:  Hilary shares some behind the scenes photos from the How I Met Your Father Finale.  Cobie Smulders and Joe Nieves appeared in cameos in the comedy as their characters from the series forerunner How I Met Your Mother

The news is out:  Hilary Duff shares some behind the scenes photos from the How I Met Your Father Finale.  Cobie Smulders and Joe Nieves appeared in cameos in the comedy as their characters from the series forerunner How I Met Your Mother

The cameos, plus some other surprises in the episode, were top secret status until the big reveal. 

Only a few people from the HIMYF crew knew about them to prevent leaks and Cobie's scenes had to be shot early in the season to accommodate her Marvel shooting schedule.

The Avengers star is reprising her role as Maria Hill for the TV series Secret Invasion. The action/adventure is expected to stream on Disney+ later this year.  

Top secret: Cobie's scenes were shot well in advance of the final episode in order to accommodate her Marvel shooting schedule. The Avengers star will reprise her role as Maria Hill for the upcoming Secret Invasion, the latest project from the Marvelverse

Top secret: Cobie's scenes were shot well in advance of the final episode in order to accommodate her Marvel shooting schedule. The Avengers star will reprise her role as Maria Hill for the upcoming Secret Invasion, the latest project from the Marvelverse 

After the HIMYF finale dropped on Hulu Tuesday, the 34-year-old actress was finally able to spill the beans about the episode to her 20.4 million followers writing, 'Keeping this quiet for months was like knowing your best friend is getting proposed to. Dear lord it was tough!!! Robin Scherbatsky! ANNNNND CARL! More importantly @cobiesmulders & the nicest @joepnieves ♥️ thanks for coming to play and adding that original magic to our show! Really meant a lot!!!'

The sitcom veteran also asked fans to weigh on what they thought of the finale. 

The post received more than 300 thousand likes, with followers leaving some lively comments including, 'I about lost my crap when I saw Robin & Carl show up!,'  and 'Loved it! Screamed when Robin popped on screen - my husband came running because he thought something was wrong lol.'

More surprises to come: The shows creators are promising more surprises in season 2.  They plan to start working on the 20 episode order they received from Hulu

More surprises to come: The shows creators are promising more surprises in season 2.  They plan to start working on the 20 episode order they received from Hulu just as soon as they wrap up their duties on the series finale of the Emmy award winning This is Us. The final episode airs in May

Fans are already looking forward to season two.

 he romcom streams on Hulu and series creators Isaac Aptaker and Elizabeth Berger are promising more surprises in the future.

The pair plan start working on 20 new episodes as soon as they wrap up their duties on This Is Us which airs its series finale in May.