Your top priority has to be your alibi, and in such situations it's a good idea to stick close to the truth.  Since you were out in the barn anyway, you agreed to say that Miss Scarlet was considering the purchase of a thoroughbred horse and asked you to come see the lot and give you a professional opinion.

The last thing you need is to look at all suspicious, so you should play the doting couple routine to the hilt.  Take very good care of Countess Emerald, who is obviously shaken up by the murder.  Unfortunately, this means that you must avoid Miss Scarlet like the plague.  She won’t be happy about it, but if she gives you trouble, point out that your affair will be over if you end up in jail.

Likewise, as tough as it may seem, be polite to Colonel Mustard.  You are not a prime suspect in the murder because you had no reason to want to kill Mr. White.  Antagonism towards Mustard may make the Inspector notice you… and start thinking more creatively about motive.

Finally, you should consider the fact that the candlestick has now vanished.  This could mean that the Inspector knows it's the murder weapon.  Your only hope is that Mrs. White will be accused of the murder instead of you.  She was known to hate her husband.