When there's a conversation between more than two people, it can get tricky. You need to make it clear who's speaking at all times by using "said" tags, as well as action tags.
"I don't know what to do," Marta said.
Jack stepped forward. "Did you try talking to him?"
"Of course I tried talking to him!" She crossed her arms and began to pace. Her long skirt whispered with every step. "He wouldn't listen."
"You'll have to find a way to make him listen," Luke said.
"But how?" Marta shook her head. "You know him--the man's as stubborn as a donkey."
"And not nearly as handsome," Luke added with a snigger.
John heaved a sigh. "We could call in Alex. He always listens to Alex--"
"Alex is in Bermuda," Luke interrupted. "Won't be back for another six weeks."
"I'm doomed," Marta said. She clenched her fists, twisting the delicate fabric of her skirt. "We'll never convince him on our own and we haven't time to wait for Alex to return."
You see how it bounces between them? You always know who is saying what, but without an endless series of "he said" and "she said".
One other thing I wanted to mention, just a quick critique of your example paragraph: when you're indicating interrupted speech, you want to use an em-dash, not a hyphen. An em-dash is basically a double dash (--), though some word processing programs will automatically turn a double-dash into a long dash, which is prettier. Technically, you can choose to put a space on either side of the em-dash or not, but it's important to be consistent. You should probably also put the tag
after the interrupting speech, not before, just so it flows better.
". . . It’s only a-”
Her husband finished the sentence. “- matter of time before there’s a coup. Eva?”
". . . It’s only a--"
"--matter of time before there’s a coup," her husband finished. "Eva?"
Hope this helps!
Edited to Add: You might want to check out the book
Dialogue by Gloria Kempton. It's been a big help to me.